Although the sun is recognized as the major terrestrial energy source, the historically abundant availability of fossil fuels and their favorable economic utilization has discouraged the widespread use of solar energy. In recent years society has recognized that the finite, and rapidly disappearing, reserves of fossil fuels require alternate energy sources for continued survival. More recently, the escalating cost of fossil fuels has made solar energy an increasingly viable alternative in selected areas of energy consumption, such as in the heating and cooling of buildings, hot water production, and the like.
A common approach to harnessing the sun's energy for home heating and cooling has been to utilize solar heating panels which directly absorb sunlight on a blackened surface, thereby causing localized heating, with the heat being transferred to a central storage area or place of need by a heat transfer medium, generally a fluid which flows over, or through, heated surfaces. Water, in part or in whole, and air are the most common heat transfer media. Such panels consist essentially of:
1. a collector or absorber plate, which absorbs radiant energy and is characterized by a relatively high absorbtivity and low reflectivity of incident radiation; PA0 2. channels through which a heat transfer medium flows in contact with the absorber plate; PA0 3. glazing, or a highly transmittive top cover plate which reduces convective heat losses; PA0 4. a frame, which is a structure which affords rigidity and mechanical integrity to the unit, separates the components, and balances their various functions; PA0 5. conduits and fixtures necessary to regulate and connect the heat transfer medium within the panel to structures outside the panel such as conduits, piping, and manifolds; PA0 6. insulation on the bottom side of the collector plate to reduce heat loss.
In the present state of the art, collectors commonly are of metal construction with their top surface often coated with a material providing high absorbtivity and low reflectivity, with the thermal conductivity characteristic of metals providing good heat transfer to the fluid. The main reason metals are used as materials of construction is their ability to mechanically and structurally withstand stagnation temperatures which can approach 400.degree. F. and attending pressures up to about 100 psi. However, the cost of such units, even through reasonably efficient, is a major obstacle to increased solar energy utilization. The goal of many prior efforts in this field has been to develop a solar heating panel of greatly reduced cost.
The use of plastics as materials of construction is generally recognized to be cost advantageous. A serious problem attending their use when glazing and insulation are incorporated is their inability to withstand temperatures and pressures to which they may be subjected at stagnation, that is, a condition where the heat absorbing fluid is not flowing. A clever approach to this problem is the use of a highly absorbtive heat transfer medium which acts as an absorber flowing in channels within a plastic structure whose top surface is highly transmittive; U.S. Pat. No. 4,134,389. Other panels utilizing plastic require bonding or laminating various layers, as in U.S. Pat. Nos. 4,146,012 and 4,159,709, which do not speak to the problem attending high temperature development. Compare also U.S. Pat. No. 4,182,308, which requires several different materials of construction exclusive of the glazing and insulation. Where only low grade heat is desired, such as in pool heaters, plastic has been found acceptable as a material of construction, for example, U.S. Pat. No. 3,868,945. In such cases it is to be clearly understood that the deliberate omission of glazing and insulation obviates problems attending high temperatures of the absorbing surface and high internal pressures, since there are large convective heat losses.
Attempts have been made to maintain the temperature of glazed solar heating panels, especially the collector element, below degradation temperatures and blow-out pressures by venting the air plenum between the collector and glazing using temperature actuated means. Thus, U.S. Pat. No. 4,046,134 describes a method of preventing overheating of plastic solar heating panels by raising the glazing, thereby venting hot air and achieving cooling by convection. In U.S. Pat. No. 4,150,659 similar cooling by convection is achieved by use of a bellows-operated hinged damper. The patentee of U.S. Pat. No. 4,219,009 describes a method wherein venting gates at opposite ends of the air plenum are opened simultaneously in response to a heat-sensing module placed within the plenum.
Because a solar heating panel of unitary construction can afford great advantages, inter alia, in cost, in quality control, and in flexibility of size and shape, such an approach is highly desirable. This is described in U.S. Pat. No. 4,170,983 where the use of ceramic as the material of construction makes the cost prohibitive, its weight unwieldy and perhaps unworkable, and fluid flow characteristics impossible to predict. However, U.S. Pat. No. 4,213,449 describes a solar heating panel of unitary construction made of blow-molded polypropylene. The patentee there describes many of the advantages accruing to such a solar heating panel, which need not be repeated here. Although the substitution of polyethylene for the polypropylene used in the aforementioned patent would be highly desirable from a cost aspect, a serious objection is that inlet and outlet connections are awkward at best because of the way the manifold ports terminate at openings in all four corners. An even more serious objection to the panel described there is that it is inoperative because there is no provision for maintaining the panel below its degradation temperature. Simply put, the panel as described would melt or rupture.
The problem presented by a plastic solar heating panel is that stagnation temperatures in the neighborhood of 300.degree. F. can be expected to be developed, and at such temperatures the most desirable materials of construction (from a cost aspect) would degrade or would otherwise undergo physical changes which would permanently destroy the structure, for example, channels in the collector element for the working fluid would blow out. A discovery leading to this invention is that the temperature of the collector can be maintained below about 220.degree. F. provided the spacing between the collector and glazing is at least two inches. In particular, such temperature maintenance is achieved without any temperature sensing means or mechanical device actuated by such means. More particularly, said temperature maintenance is achieved by vents at the top and bottom sides of the panel where the venting area is in a mathematically described relation to the collector area. Where the collector is rectangular, this relation depends only upon the length of the collector.
Thus, in one aspect the invention described herein is a method of maintaining the temperature of the collector less than about 220.degree. F. In another aspect, the invention is an insulated plastic solar heating panel of unitary construction comprising a collector, a hollow frame of upstanding walls, and glazing at least two inches above the surface of the collector member, wherein the air plenum is continuously vented to maintain interior temperatures less than about 220.degree. F.